UNDERSTANDING WATER SUPPLY

Each one of us uses about 160 litres (35 gallons) of water a day, and takes it for granted. Only in a long spell of dry weather comes an awareness that we should use it carefully. Our use is controlled by the supply system – this is how it works.

In the last 50 years the consumption of water has almost doubled. Rising standards of living have given rise to increased consumption, and a greater awareness of the need for hygiene has also played a large role in increasing the demand. Faced with this high demand, supply sources have been hard pressed to keep up.

Where it comes from

Water is supplied by the local water authority (or the ‘Undertaking’ as it is known in the plumbing trade). After falling as rain it is collected in reservoirs which are fed by streams and rivers, or is pumped from underground wells. Water varies a lot in its chemical makeup since it picks up minerals and gases as it flows. If it picks up calcium, magnesium and sodium salts it will be ‘hard’ – the menace of pipe systems. Before being distributed it is usually filtered through sand and pebble beds to remove solids and organisms, and may have chlorine added to it to ensure that it is ‘potable’ – drinkable. Fluoride is also sometimes added for the protection of teeth.

Distribution is carried out by a network of pipes starting with ‘trunk mains’ which may be as much as 610mm (24in) in diameter. These split into mains and sub-mains which run underneath streets and side streets. It is these sub-mains which are tapped by individual houses for their supply.

The house system may be ‘direct’ in which all cold water supplies are piped direct from the rising main, with the cistern only being used to supply the hot water tank. Or it may be an indirect’ system in which all cold-water supplies are taken from the cistern, with the exception of a direct supply to the kitchen sink for drinking purposes.

For water to flow through the trunk mains -and eventually into your house – it must be under a certain amount of pressure. This pressure is assisted by pumps but it is vital that somewhere in the mains system the water should reach a height in a reservoir or water tower, higher than any domestic system it has to supply. The vertical distance through which the water ‘falls’ is known as the ‘pressure head’ and without it our cisterns would never fill up without a lot of expensive additional pumping. The storage cistern also provides a pressure head inside the house, which is why it’s preferable to have it in the roof space.

The house system

The sub-main underneath the road is tapped by the ‘communication pipe’ which ends at the authority’s stop-valve. This is usually situated under the pavement about 300mm (1ft) outside the boundary of your property. The stop-valve is located at the bottom of a vertical ‘guard’ pipe – about 1 metre (39in deep – which is covered at the surface by a hinged metal cover. It should only be operated by the water authority and requires a special key to turn it. But in a real emergency you may be able to turn it yourself. In old houses it may be the only way of turning off the water supply. After this stop-valve the water enters the service pipe and from then on all pipes become your responsibility.

The service pipe continues under the wall of the property at a depth of at least 750mm (2ft 6in) to protect it from frost – though some water authorities insist that it should be 900mm (3ft) deep. As it travels under the house wall or foundation it usually goes through an earthenware pipe to protect it

OSDJ

From possible settlement which might cause it to fracture. To prevent any risk of freezing in cold weather the service pipe should not emerge above ground level until it is at least 600mm (2ft) inside the inside wall surface.

Up to about 40 years ago, service pipes were usually made of lead (in fact the word dumbing originally stemmed from the Latin word for lead Today copper and polythene are used instead The latter is particularly good as it is a poor conductor of heat and is less prone to freezing and fracture.

The service pipe

The service pipe continues under the wall near the kitchen sink, which means that it is often attached to the inner face of the outside wall. This is contrary to the recommendation that it should be attached to an inside wall, and so such a pipe should be lagged with insulation material. The pipe should also be insulated if it comes through any sub-ground floor cavity where it would be subjected to the icy blasts of winter from under-floor ventilation Again these precautions are both intended to minimise the risk of frost damage.

When the service pipe rises above the ground floor it is called the ‘rising main’ and it eventually terminates in the supply cistern, which is usually in the roof cavity The house-holder’s main stop-valve is usually found on the rising main a little way above floor level. This is the most important tap’ in the house. In any plumbing emergency when bursts or leaks occur, for example, your first action should be to turn this tap off, thus isolating the house system from the mains water supply The stop-valve should always be turned off when you go away if the house is going to be empty In old houses the location of the stop-valve may vary considerably, it may be in the cellar, under the stairs, or even under a cover beneath the front path or it may not exist at all. In which case the authority’s stop-valve is the only control

Branch supply pipes

At least one branch’ supply pipe leaves the rising main close above the stop-valve and drain tap this is to the tap over the kitchen sink. This tap must be supplied direct from the mam supply as it is supposed to provide all drinking and cooking water. Water which has been in a storage cistern is no longer considered drinkable, sometimes termed potable’, as it may be slightly contaminated by debris in the storage cistern

Other branches may be taken at this point to an outside tap. Or to a washing machine or dishwasher.

The rising main continues upwards and while its ultimate destination is the cold water storage cistern the pipework in between will vary from house to house, depending on whether a direct’ or indirect’ system has been installed.

In many areas indirect systems must be installed in new buildings, yet in Western Europe direct systems are the rule. Indirect systems have been encouraged because of the difficulty in maintaining constant mams pressure particularly at times of peak demand. Routing of most supplies through the storage cistern evens out fluctuations, and it also rules out the risk of back siphonage’ whereby dirty water could be sucked back into the mains supply though this rarely occurs. The 1976 drought in the UK provided good reason for indirect systems, since each house had an emergency supply in the storage cistern if the mains water had to be shut off

Cisterns

The ‘tank’ in your loft or attic is in fact a cistern’. Cisterns are not sealed though they should be covered – and so are subject to atmospheric pressure Tanks are completely sealed – as with a hot water storage tank – and are not subject to atmospheric pressure.

Cold water cisterns.have traditionally been made of galvanised mild steel and it is quite likely that you will find one like this in your loft. They are still available, but are not usually installed in new houses. Other materials used have been asbestos, cement, copper and glass fibre, but today the most common material is plastic, of which glass fibre reinforced polyester (GRP), polythene and polypropylene are the most common varieties.

The advantages plastics have over all other cistern materials are their lightness in weight, resistance to corrosion and flexibility. Galvanised steel is heavy and liable to corrode, while asbestos and cement are not only heavy but can become porous and are prone to accidental damage. Don’t forget the capacity of a typical cistern is 227 litres (50 gallons), and this water alone weighs nearly 0.25 tonne ton), so all cisterns must be fully supported on the joists. With rigid materials such as steel the cistern can rest across the joists, but with plastic and glass fibre a platform should be installed to support the whole area of the bottom, otherwise the material may develop local weaknesses.

Cisterns should be covered to prevent any contamination of the water Where the underside of the roof is exposed dust and dirt are liable to fall in. The top and sides should also be insulated to minimise the risk of freezing. The bottom is left uncovered to allow rising warm air from rooms below to keep the water above freezing point, and so you shouldn’t insulate the roof space under the cistern.

Cisterns were often installed before the roof was put on and if you want to replace yours, perhaps because it’s made of steel and is corroding, you may not be able to get it through the trap door. While it is sometimes suggested that a cistern should be cut up to get it out this is in fact a very heavy and arduous job in such a confined space and it would be belter to manoeuvre it to one side and leave it in the loft, installing a new cistern alongside. Modern plastic cisterns can literally be folded up so they can be passed through small loft hatches.

Pipes and taps

Water leaves the storage cistern in distribution pipes which are usually 22mm (Vnn) or 15mm (Vain) in diameter. In a direct system, supply from the cistern will usually only be to the hot water tank, and in an indirect system this link must also be direct – but other distribution pipes are used with branches to supply the other appliances – basins, baths and WC cisterns. Distribution pipes usually end in taps but in the case of a WC a low pressure ball-valve controls the flow.

The WC in an indirect system has a low pressure ball-valve because when the water leaves the storage cistern it is no longer at mains pressure but at normal atmospheric pressure which is pressing down on the surface of the stored water. This means that the higher up the house a tap or other outlet is situated the lower will be the water pressure. In practice this means that you can’t have a tap in an indirect system which is above the level of its distribution outlet from the cistern. Showers are particularly affected by this difference of pressure, and if there is not sufficient ‘head’ to ‘drive’ the shower a special pump may have to be installed.

Cold water supplied to the hot water tank is heated in two different ways again called indirect and direct systems – or, respectively. Closed and open. In the latter the cold water is circulated through the boiler, where it is heated, and returned to the tank from where it flows to tapped outlets. In the indirect system the cold water supplied never actually goes to the boiler, instead it is heated in the tank by a coiled pipe or jacket containing hot water which is continuously circulating through the boiler. In either case a pump often helps the water flow through the boiler, and supplementary or alternative heat may come from an immersion heater. If there is no boiler but only an immersion heater in the lank the system is essentially direct with the healing of the water taking place in the tank rather than in the boiler.

Draining the system

Just above the rising main stop-valve should be a drain cock. With the slop-valve turned off the drain cock can be used to drain part of the cold water system when repairs are necessary the hot water system has its own drain cock.

Sorry, comments are closed for this post.